The present invention relates to an apparatus and method for determining the percentage of solid particles in a suspension ("consistency") and in particular to an apparatus and method for using diffused radiant energy to determine the consistency of paper pulp stock.
In paper manufacturing processes, the consistency of the pulp stock is a prime factor. For purposes of this disclosure the term "stock" is intended to mean wet pulp of any type at any stage in the paper manufacturing process. Different grades and weights of paper or paper products require different consistencies of stock. In addition, the consistency of pulp for a given paper product will depend on the process point at which a pulp sample is taken. The stock consists of the comminuted wood fibers, water, and sometimes certain additives. Such pulp stock is not a homogeneous mass, but rather is a mixture which contains the foregoing materials.
Since the consistency of the stock is of prime importance in the manufacturing of paper and paper products, it is highly desirable to know its consistency at all times. Further, in order to provide a reliable and high quality paper manufacturing operation, the consistency of unknown stock must be easily ascertained. Such a determination of consistency is particularly important when it is desired to switch a paper manufacturing apparatus from one grade of product to another. Such a grade change may necessitate the employment of stock having a different consistency.
The term "consistency" as used by the paper industry designates the concentration of pulp in water on a moisture-free basis. Consistency is expressed in terms of a percentage, with the percent consistency being calculated as follows: ##EQU1## In the pulp and paper industry, the consistency of stock may vary by three orders of magnitude, for example from 0.02% to 20%.
The direct measurement of consistency is presently accomplished by numerous techniques. With practice, a person can make a subjective estimate of consistency, using his eyes. An old viscosity method for determination of consistency is known as the "pencil" method. A calibrated tapered rod about 6 inches long is dropped from a vertical position a standard distance above the stock. A reading is taken of the depth to which the rod sinks into the stock. This technique is useful in a range of from about 2.5% to about 7% consistency.
Examples of other mechanical apparatus for determining consistency are shown in U.S. Pat. Nos. 3,110,172; 3,198,006; 4,062,226; 4,276,119; and 4,301,675.
Electrical consistency meters are also known which generally utilize the electrical resistance across the stock as a measure of the stock consistency. Such apparatus is disclosed in U.S. Pat. Nos. 1,701,331 and 2,083,074. In U.S. Pat. No. 1,822,604 a pulp freeness measurement is made by placing stock samples between the plates of a condensor and measuring the dielectric constant of the stock sample.
Consistency metering devices which rely on optical properties of stock are also known. In U.S. Pat. No. 4,318,180 the particle size distribution with respect to selected fraction classes in the direction of flow of a medium is measured by passing light through the medium and detecting the light transmitted therethrough. The detection of shives (i.e., splinters) in paper pulp is the subject of U.S. Pat. Nos. 4,066,492 and 4,225,385. In the '492 patent, multiple beams and multiple photodetectors are used to enable the measurement of "breadth" as well as "thickness" of the shive. Both photodetectors in the '492 patent detect transmitted light only.
U.S. Pat. Nos. 3,518,003 and 4,171,916 utilize polarized light for determining fiber concentration and consistency in paper stock. In U.S. Pat. No. 4,040,743 back-scattered, reflected, and transmitted energy is utilized to measure pulp slurry parameters. In this patent, an optical probe is disclosed which introduces light energy into a slurry, which energy is transmitted, back-scattered and reflected orthogonally. Separate electrical signals are provided which correspond to the back-scattered, reflected, and transmitted energy. The output corresponding to the back-scattered energy is divided by the log of the output corresponding to the reflected energy to produce a measurement of consistency which is independent of the pulp brightness.
U.S. Pat. No. 3,498,719 relates to a photoelectric consistency indicator for stock. Radiant energy is projected through a flowing sample and the amount of energy transmitted through the sample is detected by a photodetector. The sample is passed through a transparent tube constituting a lens having a given focal length. The photodetector is located at a point removed from the focal length to receive a low-resolution blurred image of the sample in transit. A second photodetector is directly illuminated and provides a reference for comparison with the response of the first photodetector. The two photodetectors are interconnected in an arrangement known as an electrical bridge, such that an electrical imbalance in the bridge is indicative of the consistency of the stock.
U.S. Pat. No. 3,962,581 discloses an infrared consistency meter wherein light from a low, constant power incandescent light source is reflected from the suspension flow stream, filtered to block passage of wavelengths less than 0.70 microns, and detected by a photocell. The apparatus can be configured to emphasize either a direct relationship between the photocell output and consistency changes or an inverse relationship thereof.
Each of the consistency meters or methods for determining consistency disclosed in the above patents can provide inaccurate results under certain conditions. Variables which can affect the accuracy of such consistency meters and methods are the type of pulp (e.g., long fiber bleached kraft, short fiber bleached kraft, short fiber unbleached ground wood), slurry flow rate, temperature, pulp freeness, pressure, ash content, and pH. It would be advantageous to provide an apparatus and method for measuring consistency which is linear through a wide range, e.g., from less than 0.01 through 15% consistency.
Through the use of diffused radiant energy, and the detection of forward-scattered and back-scattered energy in a suspension being measured, the present invention provides such an apparatus and method.